Synaptic plasticity is essential for the development of brain, learning and memory. Hebbian-type plasticity such as long-term potentiation and long-term depression is rapid and synapse-specific modification. In contrast, homeostatic plasticity involves global modification of synapses, operates over longer timescales, and is believed to be crucial for the maintaining and orchestrating neuronal network function. Hebbian-type plasticity is mediated mainly by the trafficking of AMPA receptors but not much is known for the mechanisms of homeostatic plasticity. Recently, activity-dependent protein turnover at the synapses by ubiquitin-proteasome system has emerged as crucial mechanisms associated with various types of synaptic plasticity including homeostatic plasticity. However, it is unknown how activity orchestrates concomitant ubiquitination/degradation and recruitment of specific group of proteins at synapses. Among the activity-regulated proteins, GKAP is one of the major scaffolding proteins in the postsynaptic densities and provides a molecular link for PSD-95/NMDA receptor complex and Shank/Homer. Our preliminary studies suggest that activity controls the recruitment and removal of GKAP from synapses, both through Ca2????dependent protein kinase II (CaMKII). Further, we found that the activity-dependent turnover of GKAP is required for synaptic scaling in hippocampal neurons. In this proposal, we will investigate the molecular mechanisms by which CaMKII controls ubiquitination/degradation or recruitment of GKAP to synapses, and the functional significance of the GKAP turnover at the synapses in various types of synaptic plasticity. Aim 1 will map the CaMKII phosphorylation site(s) and ubiquitinated lys site(s) that induce ubiquitination of GKAP, using a combination of mutagenesis and biochemical assays. Aim 2 focuses on understanding the role of DLC, MyoV, and CaMKII for GKAP recruitment to synapses by molecular genetic approaches. We will also perform real-time imaging to understand dynamic GKAP trafficking with greater spatio-temporal resolution. Aim 3 will assess the functional significance of GKAP removal/recruitment at synapses for the activity-dependent modification of synapse compositions and various forms of synaptic plasticity, by using GKAP mutants lacking the activity- dependent turnover. Since aberrant synaptic plasticity is implicated for a variety of neurological and neuropsychiatric diseases, the proposed studies will not only allow us to gain novel and fundamental insight into the molecular mechanisms for long-lasting changes in synapse compositions but also are relevant to these brain diseases.